a) Field of the Invention
The present invention relates to a surface light source device which is used in back light systems of liquid crystal display devices.
b) Description of the Prior Art
FIG. 1 is a cross-sectional view showing a conventional surface light source device. In FIG. 1, the numerical reference 1 represents a linear light source such as a cold cathode ray tube, 2 a light conducting member made of a transparent material, 3 a light diffusing member and 4 represents a light reflecting member. In the surface light source device shown in FIG. 1, a light emitted from the light source 1 is made incident upon a light incident edge surface 2a of the light conducting member 2 and then introduced into the member 2. The light is transmitted inside of the member 2 to an opposite edge surface 2d side, being totally reflected by a front surface (a light emitting surface) 2b and a rear surface 2c of the light conducting member 2. On the rear surface 2c of the light conducting member 2, a coarse surface is arranged, a multiplicity of conical or polygonal unevenness is provided, or a pattern constituted of a multiplicity of fine diffusing portions is formed by printing to change a part of returning light, which is directly reflected by the rear surface 2c or passed through the rear surface 2c and then reflected by the light reflecting member 4, into diffusion light. The diffusion light is emitted from the front surface 2b (light emitting surface) of the light conducting member 2; and then passes through the light diffusing member 3 to form a surface light source of the liquid crystal display device.
If the distribution of the coarse surface, the heights and the top angles of the conical or polygonal-shaped unevenness, or the area and distribution density of the fine diffusing portions of the printing pattern, which is formed on the rear surface 2c of the light conducting member 2, is varied in a suitable manner, a uniform brightness distribution of the light emitted from the light emitting surface 2b of the light conducting member 2 can be obtained.
However, in the above-stated conventional surface light source device using a light conducting member, when the distribution of the coarse surface formed on the rear surface of the light conducting member, the heights of the conical or polygonal-shaped unevenness or inclined angles of the top portions thereof, or the variation of the distribution density of the printing pattern is not suitable, it is impossible to obtain a uniform brightness distribution of the light emitted from the light emitting surface, i.e. the light emitted from the light diffusing member. Therefore, in the conventional device, an arrangement of the above-said coarse surface distribution and its variation is very important to obtain a uniform surface light source. However, it is not so easy to obtain an optimum coarse surface distribution and its variation, and thus difficult to form the suitable coarse surface distribution, etc. on the rear surface of the light conducting member as designed first.